An option to prioritize selected objects.
This has many use cases, here is one examples to - hopefully - make the function more clear:
If you have a stack of objects which overlap, the selected object will be prioritized, meaning you can move and scale it until you »deactivate« the prioritization or you deselect the object. You don't deselect this object if you click move the object with your mouse, even if another object is in front if it.
This can also be helpful when you work with type, where a transparent bounding box around the type often leads to accidental deselection of objects which are behind the text.

It's comparable to an object in photoshop when you're in transform mode (hitting command + T to transform). The object can be moved and scaled while it's in the transformation mode without you accidentally selecting another object = deselecting the current one.

Also very helpful when you work wirth text. If you work with text and it's selected, you don't accidentally deselect because you clicked 1px next to the bounding box of the text.